In aircraft cooling systems, conditioned supply air to an aircraft cabin may be recirculated and mixed with outlet air from several air conditioning packs via a mix manifold and ductwork system. The ductwork typically includes separate ducts coupled to opposite sides of a lower portion of the mix manifold to convey recirculated air from different zones of the aircraft cabin for mixing with the air from corresponding air conditioning packs. Each of these ducts may be coupled to the mix manifold such that air flow is directed tangentially into the mix manifold or toward the center of the mix manifold, depending on various design considerations. In single or double air conditioning pack operation that involves tangential-directed airflow into the mix manifold, air may still be rotating in the upper portion of the mix manifold. This, in turn, may lead to rotating airflow in an outlet branch of the mix manifold that may cause air pressure oscillations. The result may be strong vibration of the ductwork and manifold, which is undesirable as it may cause premature failure of the ductwork. In addition, these air pressure oscillations may also periodically change zone-to-zone airflow balance in the ductwork.